Michael Kurland has a written a series of novels with Professor Moriarty as the central character, and Holmes as a bit of a pest. The series starts with "The Infernal Device".
You might also like Randall Garrett's Lord Darcy stories; which features a Holmes-like aristocrat detective in an alternative history which magic = technology; darcy's aide is a forensic sorcerer. You can get a very inexpensive Darcy omnibus (DRM-free in multiple formats) for $4.00 at
http://www.webscription.net/s-48-randall-garrett.aspx